The present invention relates to ultrasound imaging and, more particularly, to ultrasound imaging using phased-array transducers. A major objective of the present invention is to provide for improved real-time ultrasonic imaging where the image depth exceeds the depth of field of the ultrasonic transducer.
Ultrasound imaging is widely used for non-invasive investigation of a subject. Medical applications include cardiac monitoring and fetal monitoring. A typical ultrasound imaging system comprises a base module and a probe. The base module generates an electrical pulse burst which is converted to an ultrasound acoustic wavefront by a transducer in the probe. When the probe is pressed against a subject, the acostic wavefront is transmitted into the body and is reflected to different degrees wherever it encounters a change in ultrasonic impedance (the product of density and propagation velocity) at tissue or bone boundaries. Echoes from different boundaries reach the transducer at different times depending on the distance of the respective boundaries from the transducer. The transducer converts the echoes into time-varying electrical signals which are processed to form a video presentation of the subject being imaged.
Fixed focus ultrasound systems are known which employ spherical or parabolic transducers. Applying an electrical pulse to such such a transducer generates an ultrasound wavefront which converges at the transducer's focal point. The focal range for fixed-transducer ultrasound systems is its depth of field. The sensitivity, i.e., ability to detect relatively weak echoes, of such a system is related to the aperture area available for detecting these echoes. Greater sensitivity can be obtained by using a larger aperture.
As in optical systems, larger apertures are associated with greater energy gathering ability but shallower focal ranges. Thus, greater sensitivity can be achieved at the expense of focal range. Greater sensitivity can also be obtained by increasing the power transmitted into a subject of interest; however, the transmit power is constrained by subject welfare considerations as codified in regulations of the Food and Drug Administration (FDA). Thus, in practice, for fixed focus systems, there is a tradeoff between sensitivity and focal range.
A variable-focus transducer can provide a focal range greater than its instantaneous depth of field. Thus, higher sensitivity can be obtained along with a greater focal range. While mechanical deformation of a transducer to vary its focal depth is conceivable, it is not practical. An "electronic deformation" is accomplished in a phased-array transducer, a development derived from radar technology. A phased-array transducer comprises multiple transducer elements in one of several spatial configurations. By introducing variable relative delays between the electrical channels associated with respective transducer elements, the focal depth of the array can be adjusted.
There are three basic types of phased-array transducers: planar, linear and annular. Planar arrays comprise a two-dimensional array of "point source" transducer elements. Linear arrays comprise a one-dimensional array of linearly-extending "line source" transducer elements. Annular arrays comprise a radially extending array of circular transducer elements, i.e., coaxial rings.
All three array types provide electronic focusing. Planar and linear arrays can be steered electronically, while annular arrays are steered mechanically. Planar arrays provide full electronic control of steering and focusing, as well as the ability to resolve in three dimensions. They have not been used widely due to the large number of transducer elements and corresponding signal processing channels required to achieve a given resolution. For example, a planar array which can focus in azimuth would require 500 to 1,000 transducer elements and channels to achieve the resolution attainable by a twelve-element annular array. To steer in azimuth would require 5,000 to 10,000 transducer elements. Linear arrays provide electronic steering and focusing, but fail to resolve in the azimuthal direction. This disadvantage limits their usefulness for many applications.
Annular arrays resolve in three dimensions and have relatively modest signal processing requirements. Mechanical steering can be slower than electronic steering. Typically, an annular array is constantly wobbled fast enough so that a human operator does not perceive a significant delay in the acquisition of image data from two different angular positions. Mechanical steering introduces some limitations in that steering positions are acquired serially rather than by direct "random" access, as is provided by electronically steered arrays. Another disadvantage is that the transducer array is moving between the time a wavefront is transmitted and the time the last echo of interest is received. Complex signal processing can be required if it is deemed necessary to remove the resulting distortion. In practice, annular arrays provide cost effective ultrasonic imaging.
In an annular array transducer system, as in other phased array systems, a burst comprising parallel trains of one or more electrical pulses is generated. Each pulse train is directed to a respective transducer ring on a respective transmit channel. Each transducer ring receives its respective pulse train, which it converts to an ultrasound pulse. The ultrasound pulses generated at the different transducer rings combine to define an ultrasound wavefront. If the elements of the array collectively define a spherical or parabolic shape, the wavefront can converge at the geometric focal point of the array. An image obtained using such a wavefront will be sharpest at the focal point and increasingly out of focus at shallower and deeper positions along the same steering axis. For a given application, "acceptable" focus is obtained within a geometric depth of field, which includes the geometric focal point.
A focal point other than the geometric focal point is achieved by introducing relative delays in the pulse trains. Relatively longer delays applied to the inner transducer rings result in a wavefront which converges nearer to the array, i.e., a shallower focal point is effected. Shorter delays applied to the inner transducer rings effect a greater focal depth. The delays are selected to compensate for the path differences between respective transducer rings and a targeted focal point. Thus, focal depth can be varied by adjusting the relative delays between inner and outer transducer rings.
Relative delays can be introduced both in the transmit channels and in the receive channels. Generally, the focal point for reception should be the same as the focal point for transmission. This is qualified by the availability of dynamic receive focus, which has no direct counterpart for transmission. The echoes resulting from a single transmit burst are received chronologically according to the distance between the respective boundaries generating the echos from the transducer array. A receiver implementing dynamic receive focus, adjusts the relative channel delays so that the instantaneous focal point tracks the position from which echoes are received at that instant. Dynamic receive focus optimizes focus through the reception of the echoes from each burst.
Dynamic receive focus takes advantage of the fact that burst's energy is distributed in space and time before echo reception. There is no comparable time distribution of energy at the time the pulse is transmitted. The duration of a pulse burst is too short to permit viable changes of focus during a pulse burst. Accordingly, the advantages provided by dynamic receive focus have not been transferred to the transmitter. Therefore, the instantaneous focal range of an ultrasound system remains limited by the transducer's instantaneous depth of field.
Multiple steering sweeps can be used to view an area deeper than the depth of field. The transducer is rotated over a desired steering range while the transmitter selects delays corresponding to a first focal depth. This results in an image which includes a band (defined by the depth of field) of acceptable focus. A second steering sweep provides a second band of acceptable focus. These bands can be juxtaposed to provide a range of acceptable focus greater than the transducer's depth of field.
There are two problems with images obtained by juxtaposing successively acquired bands of acceptable focus: seams in the image and the lack of real-time feel. In most medical applications, the object of interest, e.g., a fetus or heart, is moving. Since an ultrasound image is acquired over time, some object motion occurs during imaging. As long as neighboring points represent neighboring instants, the collective image remains coherent and useful. However, when bands of acceptable focus are juxtaposed neighboring points on either side of the boundary between the bands represent the object at significantly different times. The band boundary becomes a visual time warp and impairs interpretation of the image.
An ultrasound operator generally moves the ultrasound probe about to view different areas within a subject. The movement can serve to locate an object requiring further study or it can serve to track an extended feature such as an artery. Ergonomic studies have shown that an ultrasound image must be updated within a certain time for the operator to feel as though the imaging is occurring in real time. If the imaging updating is slower than about 7-11 Hz, the eye-probe coordination of the operator is broken. This can impair investigation, and this impairment is exacerbated by the operation aggravation which usually accompanies the loss of real-time feel.
If two or more steering cycles are required to complete an image, there is a challenge in obtaining the multiple sweeps within the limit required for ergonomic updating. The steering speed is limited by the need to wait for echoes before firing pulses for the next steering position. Mechanical steering inevitably imposes some time loss between steering cycles.
Planar arrays are not restricted by mechanical steering but are faced with the delays in processing many times the information handled by annular arrays. Planar arrays must also address the problem of time discontinuities at band boundaries. All ultrasound systems are restricted by the time of flight of ultrasound in the body.
What is needed is an ultrasound system which provides full focus images of areas of depth greater than the instantaneous depth of field of the transducer aperture. This would permit high sensitivity and extended focal ranges to be combined more advantageously. The full focus images should be relatively free of temporal discontinuities. Furthermore, the imaging system should permit full range, full focus imaging within the ergonomic limits required for real time feel.